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Power Meter and Sensor Tutorial
Power Meter Measurements
Thorlabs' power meter consoles have the ability to recognize the connected sensor type as well as its responsivity. The measured output signal depends on the sensor type. In addition, Thorlabs' Power Meter Consoles are capable of displaying the photocurrent or voltage delivered by the sensor. The equivalent of the measured value is also provided at the analog output.
Photodiode sensors deliver a current that depends on the optical power and wavelength of the incident beam. This current is fed into a transimpedance amplifier, which outputs a voltage that is proportional to the input current. The photodiode's individual calibration data (responsivity vs. wavelength) is saved to the EEPROM in the sensor's DB9 connector. Since the photodiode's responsivity depends on the wavelength, the actual wavelength needs to be entered to obtain an accurate measurement. The power meter console determines the responsivity for the input wavelength from the connected sensor and calculates the optical power from the measured photocurrent.
Thermal sensors deliver a voltage that is proportional to the input optical power. The sensor's individual responsivity is saved to its EEPROM and read by the console. Based on the measured sensor output voltage and its responsivity, the console calculates the optical power incident upon the sensor.
Energy sensors are based on the pyroelectric effect. Hence, they deliver a peak voltage that is proportional to the pulse energy. The individual sensor's responsivity is saved to its EEPROM. If a pyrosensor is recognized, the console automatically detects the peak voltage and calculates the pulse energy from the responsivity.
How a Photodiode Works
Typical Properties of Photodiodes
A photodiode (PD) is a semiconductor p-n junction. When photons of sufficient energy hit the p-n junction, they excite electrons, resulting in a current in the junction. Photodiodes can be operated in photovoltaic or photoconductive mode. In photovoltaic mode, the anode and the cathode are connected to a load so the photodiode delivers a current. In photoconductive mode, the photodiode is biased in the reverse direction, and the reverse current depends on the incident optical power. Reverse biasing significantly decreases the reaction time to incident photons. Consequently, the photoconductive mode is used in high-speed photodetectors. The temperature dependency of the PD current, however, is a downside of the photoconductive mode.
In power measurement applications, photodiode sensors are used in photovoltaic mode. The anode and cathode are connected to the input of a transimpedance amplifier that converts the photocurrent into a voltage. A photodiode can deliver a photocurrent up to several mA. The responsivity of a photodiode is the ratio of delivered current to incident optical power and is usually stated in A/W. This responsivity depends significantly on the material the photodiode is made from as well as the wavelength of the incident optical power. The maximum output current of a photodiode is limited to the linear part of the characteristic IPD = f (Poptical) curve, or in other words, by the saturation.
To extend the maximum measurable power to tens of milliwatts, an attenuator is placed in front of the photodiode. Usually this attenuatior is a neutral density (ND) filter. Like the responsivity, the optical density of an ND filter is also wavelength dependent. Over time, a photodiode's responsivity will change as a result of aging. For very small beam diameters, variations in photodiode sensor uniformity may lead to discrepancies in the output current. If the PD sensor's active area is overfilled, incorrect readings may result.
How an Integrating Sphere Sensor Works
The inner surface of an integrating sphere has a high reflectance over a wide wavelength range. It is manufactured from PTFE based high reflective bulk material and is resistant to heat, humidity, and high levels of radiation.
The incoming light beam is diffusely reflected from the inner surface of the sphere, which creates an even intensity distriution over the entire sphere surface. A photodiode is recessed inside a detector port to avoid exposure from incidental light. Therefore, the attenuation of the light detected by the photodiode is proportional to the active area of the detector divided by the surface area of sphere. If dPD is the diameter of the detector's active area and dIS is the inner diameter of the integrating sphere, then the attenuation can be written as:
How a Thermal Power Sensor Works
Thermal power sensors incorporate thermocouples and utilize the principles laid out in the thermoelectric effect (aka the Seebeck Effect), which state that any conductor subjected to a thermal gradient generates a voltage. Hence, if a temperature difference is present between two surfaces, the temperature gradient will generate a voltage difference between these two surfaces. This process can be thought of as the inversion of the Peltier effect.
In a thermal power sensor, the power of the incident laser beam is absorbed by the incident surface of the thermocouple and converted into heat. The other surface of the thermocouple remains cold, as it is thermally connected to the radiator of the sensor. The temperature gradient between the two surfaces depends on the incident optical power. Therefore, the generated voltage between the hot and cold surfaces is proportional to the incident power.
The conversion of optical power into a measurable voltage depends on the capability of the sensor surface to absorb the optical power and convert it into heat. In order to increase absorption, the sensing surface is coated. Preferably, the coating should be wavelength independent (i.e., the absorption efficiency is independent of wavelength) and should have a high damage threshold (i.e., capability to withstand high optical power densities).
Wavelength Dependency of a Thermal Sensor
How a Pyroelectric Sensor Works
Pyroelectric sensors can be classified as thermal detectors. They permit the direct transformation of an energy pulse into a voltage pulse independent of the wavelength of the incident radiation. These detectors are built on a coaxial basis and are therefore extremely insensitive to interference from electromagnetic radiation from sources such as a pulsed gas laser.
Each detector is coated with a black absorption layer providing nearly constant absorption in the 185 nm to 25 µm range. The comparatively high sensitivity of these detectors is particularly beneficial. Without an additional amplifer, these sensors permit laser pulse measurements in the µJ range due to their insensitivity to interference.
The maximum pulse repetition rate depends on the internal capacitance of the detector as well as the load resistor. All detectors can be directly connected using BNC sockets to the 1 MΩ input of an oscilloscope. A small load resistor (100 kΩ load resistors are included) can be used to obtain the highest pulse repetition rate, making it possible to measure repetition rates up to 100 Hz. The corresponding sensitivity of the sensors is also specified.
Calibration of a Power Meter Console
Thorlabs' PM100x and PM320E Power Meter Display Units are calibrated by applying a known current from a traceable calibrated source (Keithley 2611A Source Meter) on each gain stage of the photodiode input and a known voltage on each gain stage of the thermal and pyroelectric sensor input. Each gain stage is adjusted at the zero and 90% full scale levels. The analog output is not adjusted but measured during the adjustment and calibration procedure using a traceable multimeter (Agilent 34401). A PC controls the automated setup, acquires the data, and stores the measured values.
Calibration of a Photodiode Power Sensor
The variation of the PD's responsivity and of the ND filter's attenuation over the operating wavelength range causes the need to calibrate the photodiode in combination with the attenuator. Thorlabs uses a NIST-traceable calibration procedure. The responsivity vs. wavelength of a NIST- traceable reference diode is compared to that of the sensor (i.e., detector) under test (DuT), The responsivity of the sensor being tested is then calculated from the well-known responsivity curve of the reference diode.
Block Diagram of Thorlabs' Photodiode Sensor Calibration Setup
The optical signal is generated by a suitable white light source [deuterium lamp for 200 - 400 nm or a quartz tungsten halogen lamp (indicated as QTH Lamp in the schematic above) for 350 - 2500 nm], followed by a chopper and a CornerstoneTM 260 1/4m monochromator. The monochromator allows the wavelength of the optical signal incident to the REF / DuT to change with a step size of 5 nm. The measured photocurrent is applied to the input of a high gain (40/50 dB) transimpedance amplifier, and its output voltage is measured by a Lock-In Amplifier. The advantage of a Lock-In Amplifier is that noise and DC offsets are efficiently suppressed and only the wanted signal is measured. The required alternating signal is provided by chopping the light source prior to entering the monochromator; the chopper is synchronized with the Lock-In Amplifier. The entire light path, from illuminator to the DuT, is shielded from environmental light with blackout material.
A PC controls the entire calibration process via a LabVIEW application, programs the EEPROM of the DuT with the measured calibration data, and creates the Certificate of Calibration. For quality management purposes, each calibration scan is saved and can be recalled at any time if the sensor type and its serial number are known. The light source's overall operation time is counted by an operation-hour counter in order to follow the manufacturer's exchange instructions.
Calibration of a Thermal Power Sensor and Traceability
The goal of the calibration is to characterize the spectral responsivity of the sensor under test (SuT). The use of PTB- and NIST-calibrated references makes the calibration of Thorlabs' power sensors traceable. The responsivity is defined as the proportionality factor between the input optical power and the output voltage of the SuT. As described above, the responsivity depends on the wavelength, due to the wavelength dependency of the absorbance; thus, the measurements need to be conducted over the complete range of the SuT. The calibration consists of two main steps: measurement of the reflectivity over a given spectral range and measurement of the absolute responsivity at a known wavelength.
Reflectivity of the Coating from 266 - 1064 nm
A broadband light source is fed into an integrating sphere. The output port of the integrating sphere is covered with a NIST-traceable certified 20% reflectance standard (REF). A spectrometer is connected to the detector port of the integrating sphere (aka "baffled port"), using an optical fiber.
The spectrum is measured in the wavelength range from 266 - 1064 nm and the spectral intensity distribution is used as a reference.
In the next step, the reflectance standard (REF) is replaced by the sensor under test (SuT). A second spectrum is measured. The spectral reflectivity of the sensor's absorber surface is the difference between the two measured spectral intensity distributions.
Absolute Responsivity Measurement at a Certain Wavelength
The laser (fiber laser, 1064 nm, output power up to 100 W) is warmed up for 20 minutes to ensure maximum stability. The beam is split into two paths using a 96:4 wedge beamsplitter. Depending on the measurement range of the SuT, the 4% or the 96% path is chosen. In the left figure above, the calibration of a low-power sensor is illustrated: The 96% path is being monitored by a secondary reference sensor. This monitoring is necessary to observe possible power variations during the measurements. The NIST/PTB Reference sensor is placed into the 4% path. The laser output power is set and monitored, and the output voltage of the reference sensor is averaged over 60 seconds. This procedure is subsequently repeated 5 times.
The reference sensor is then replaced by the SuT and the above procedure is repeated. The output voltage of the sensor under test is measured and averaged over 60 seconds, and 5 subsequent measurements are conducted. The correlation between the measurements allows the calculation of the responsivity of the sensor under test at the lasing wavelength (1064 nm) through an unbroken chain of comparisons (traceability) with the NIST/PTB reference. This value will be used as the anchor point for the responsivity.
The responsivity for the remaining operating wavelength range (up to 25 μm, depending on the sensor type) is extrapolated. Each coating has a theoretical reflection curve for its entire measurement range. This theoretical curve is fitted onto the traceably determined responsivity between 266 nm and 1064 nm. Then, the responsivity of the sensor under test can be calculated up to the maximum specified wavelength. NIST/PTB traceable calibration of the sensor under test at longer wavelengths is available upon request.
All our equipment used for calibration is traceable to both national and international standards and is periodically recalibrated to maintain the required standards.
Click to Enlarge
The PM160 wireless power meter, shown here with an iPad mini (not included), can be remotely operated using Apple mobile devices.
This tab outlines the full selection of Thorlabs' Power and Energy Sensors. Our photodiode and thermal sensors are compatible with all of Thorlabs' current line of power meter consoles, while our pyroelectric sensors are compatible with all of our current power meter consoles except for the PM100A Analog Power Meter Console. In addition to the power and energy sensors listed below, Thorlabs also offers all-in-one, wireless, handheld power meters and compact USB power meter interfaces that contain either a photodiode or a thermal sensor, as well as power meter bundles that include a console, sensor head, and post mounting accessories.
Thorlabs offers three types of sensors:
Power and Energy Sensor Selection Guide
There are two options for comparing the specifications of our Power and Energy Sensors. The expandable table below sorts our sensors by type (e.g., photodiode, thermal, or pyroelectric) and provides key specifications.
Alternatively, the selection guide graphic further below arranges our entire selection of photodiode and thermal power sensors by wavelength (left) or optical power range (right). Each box contains the item # and specified range of the sensor. These graphs allow for easy identification of the sensor heads available for a specific wavelength or power range.
|(Click Photo to Enlarge)|
|Description||Analog Power Meter Console||Digital Power and Energy Meter Console||USB Power and Energy Meter Interface||Touchscreen Power and Energy Meter Console with Multi-Touch||Dual-Channel Benchtop Power and Energy Meter Console|
|Compatible Sensors||Photodiode and Thermal||Photodiode, Thermal, and Pyroelectric|
(H x W x D)
|7.24" x 4.29" x 1.61"
(184 mm x 109 mm x 41 mm)
|7.09" x 4.13" x 1.50"
(180 mm x 105 mm x 38 mm)
|3.67" x 2.38 " x 1.13"
(93.1 mm x 60.4 mm x 28.7 mm)
|5.35" x 3.78" x 1.16"
(136.0 mm x 96.0 mm x 29.5 mm)
|4.8" x 8.7" x 12.8"
(122 mm x 220 mm x 325 mm)
|External Temperature Sensor Input
(Sensor not Included)
|-||-||-||Instantaneous Readout and Record Temperature Over Time||-|
|External Humidity Sensor Input (Sensor not Included)||-||-||-||Instantaneous Readout and Record Humidity Over Time||-|
|GPIO Ports||-||4, Programmable||-|
|Source Spectral Correction||-||-||-||-|
|External Trigger Input||-||-||-||-|
|Type||Mechanical Needle and LCD Display with Digital Readout||320 x 240 Pixel Backlit Graphical LCD Display||No Built-In Display
Controlled via GUI for PC
|Protected Capacitive Touchscreen with Color Display||240 x 128 Pixels
Graphical LCD Display
|Dimensions||Digital: 1.9" x 0.5"
(48.2 mm x 13.2 mm)
Analog: 3.54" x 1.65"
(90.0 mm x 42.0 mm)
|3.17" x 2.36"
(81.4 mm x 61.0 mm)
|-||3.7" x 2.1"
(95 mm x 54 mm)
|3.7" x 2.4"
(94.0 mm x 61.0 mm)
|Refresh Rate||20 Hz||Dependent on PC and Settingsa||10 Hz (Numerical)
25 Hz (Analog Simulation)
|Mechanical Analog Needle||-||-||-||-|
|Simulated Analog Needle||-||Requires PCc|
|Bar Graph||-||Requires PCc|
|Trend Graph||-||Requires PCc|
|Type||-||SD Card||-||NAND Flash||-|
|Size||-||2 GB||-||4 GB||-|
|Battery||LiPo 3.7 V 1300 mAh||-||LiPo 3.7 V 2600 mAh||-|
|External||5 VDC via USB or Included AC Adapter||5 VDC via USB||5 VDC via USB||Selectable Line Voltage: 100 V, 115 V, 230 V (±10%)|
|(Click to Enlarge)|
|Detector Type||Slim Si Photodiode||Thermal||Thermal|
|Wavelength Range||400 - 1100 nm||190 nm - 10.6 µm||190 nm - 20 µm|
|10 nW - 2 mW
Up to 200 mW
w/ ND Filter
|100 µW - 2W||10 mW - 70 Wa|
|Optical Power Resolution||100 pW
(10 nW w/ Filter)
|10 µW||1 mW|
|Aperture||Ø9.5 mm||Ø10 mm||Ø25.2 mm|
|Distance to Sensor
From Top Surface of Housing
(4.2 mm w/ Filter)
|2.6 mm||4.5 mm|
SM05 (0.535"-40) Threads
SM1 (1.035"-40) Threads
SM1 (1.035"-40) Threads
Thorlabs offers two all-in-one handheld, wireless power meters with built-in graphical Organic LED (OLED) displays. The PM160 wireless power meter uses an ultra-slim silicon photodiode sensor, similar to our slim photodiode sensors. The sensor is designed to detect optical powers between 10 nW and 2 mW at wavelengths between 400 and 1100 nm. This device comes with a slideable neutral density (ND) filter that can be used to increase the maximum measurable optical power from 2 mW to 200 mW. The power meter will automatically detect the presence of the filter over the sensor to display the correct power measurement.
The PM160T wireless power meter contains the same thermal detector used in the S401C high-sensitivity thermal sensor head, making it suitable for broadband power measurements. It can detect powers between 100 µW and 2 W for wavelengths from 190 nm to 10.6 µm.
The PM160T-HP high-power wireless power meter features a Ø25.2 mm sensor that is designed to detect optical powers between 10 mW and 70 W at wavelengths between 190 nm and 20 µm. It is well suited for use with a wide variety of broadband and MIR light sources, including LEDs, SLDs, and MIR quantum cascade lasers.
These wireless power meters can fit into tight spaces and pick up an incident beam from any direction. The ultra-slim sensor end of the device connects to the handle with a joint mechanism that allows 270° degrees of rotation. When rotating the sensor arm, there is a detent every 90° and enough friction in the joint mechanism that the sensor arm can be positioned at any angle within the allowed rotation range.
The output of the sensor is converted to a calibrated optical power measurement that can be displayed on the built-in OLED screen. The power meter can be controlled locally by the four buttons located on the sides of the OLED display's housing. Alternatively, data can be transmitted via Bluetooth or USB to an external device. A USB stick with an application for acquiring and displaying data on PCs running Windows and on Mac computers is included with each power meter. They can also be operated from Apple mobile devices via bluetooth using the Optical Power Meter App, available free of charge at the Apple App Store.
The table to the right summarizes some of the key features of our handheld power meters. More details can be found here.
Thorlabs has integrated some of our most popular sensor head formats with a compact USB power meter interface that can be operated using a computer running one of Thorlabs' power meter software packages. Sensor head options include our standard photodiode sensors, a slim-format photodiode sensor, integrating spheres, and a thermal sensor. A 1.5 meter cable connects the sensor head to the power meter electronics, which are contained in a compact 65 mm x 20 mm x 10 mm (2.60" x 0.80" x 0.40") USB package. These extremely compact power meters feature similar internal electronics to Thorlabs' PM160 and PM160T Wireless Power Meters and can be controlled via a computer running the Optical Power Monitor Software.
The wavelength and power ranges of available stock options are outlined in the table below. Each sensor head is individually calibrated and is shipped with a NIST- and PTB-traceable calibration certificate. The calibration and identification data is stored in the power meter's USB stick housing and applied to calculate accurate power measurements. If you are interested purchasing a Thorlabs sensor head with a compact USB power meter interface that is not offered below, please contact tech support for information on custom options. Four a complete list of sensor and console options, see the Sensor Selection and Console Selection tabs, respectively.
|Detector Type||Si Photodiode||Ge Photodiode||Slim Si Photodiode,
Slideable ND Filter
|Wavelength Range||400 - 1100 nm||700 - 1800 nm||400 - 1100 nm||350 - 1100 nm||800 - 1700 nm||190 nm - 10.6 µm|
|50 nW - 50 mW||500 nW - 500 mW||50 nW - 40 mW||500 pW - 5 mW
(Up to 500 mW with Filterb)
|1 µW - 500 mW||100 µW - 1 W (3 Wa)|
|Optical Power Resolution||<1 nW||<10 nW||<2 nW||<100 pW
(10 nW with Filterb)
|<1 nW||<1 µW|
Thorlabs also offers bundles that combine one of our popular PM100D, PM100A, or PM400 power meter consoles with a C-series photodiode or thermal sensor, and a post assembly consisting of a TR2 Ø1/2" post, a PH2 post holder, and BA2 base. Available bundle options are outlined in the table below. For more details on our complete selection of sensor and console options, see the Sensor Selection and Console Selection tabs. Additional information on available power meter bundles can be found here.
|Power Range||Sensor Type
|PM120VA||200 - 1100 nm||50 nW - 50 mW||Si (S120VC)||Analog
|PM120D||400 - 1100 nm||50 nW - 50 mW||Si (S120C)||Digital
|PM121D||500 nW - 500 mW||Si (S121C)|
|PM130D||500 pW - 500 mWa||Slim Si (S130C)|
|PM122D||700 - 1800 nm||50 nW - 40 mW||Ge (S122C)|
|PM125D||0.19 - 20 µm||2 mW - 10 W||Thermal (S425C)|
|Power Range||Sensor Type
|PM400K1||400 - 1100 nm||50 nW - 50 mW||Si (S120C)||Touchscreen Digital
|PM400K2||500 nW - 500 mW||Si (S121C)|
|PM400K3||500 pW - 500 mWa||Slim Si (S130C)|
|PM400K4||700 - 1800 nm||50 nW - 40 mW||Ge (S122C)|
|PM400K5||0.19 - 20 µm||2 mW - 10 W||Thermal (S425C)|
Beam Size Compared to the Sensor's Active Area
Most sensors do not provide completely uniform response over their active area (with the exception of integrating sphere sensors, which incorporate this feature as part of their design). Thus, to overcome these uniformity issues, the incident beam should have a diameter that fills at least 10% of the sensor's active area. However, it is also important not to overfill the sensor (i.e., the beam size in the plane of the sensor's active area must not exceed the size of the active area). For high power or high energy beams, we recommend selecting a detector size that is about 20% to 30% larger than the beam diameter.
The beam size will also affect whether a source is within the maximum allowed power and energy density specifications for the sensor. These maximum ratings are included in the specifications for each sensor. Please note that if your beam does not have a flat spatial intensity distrubution, the maximum power or energy density specification must not be exceeded in the brightest region of the beam. For example, a beam with gaussian intensity distribution should not have a power (energy) density at the center of the beam that exceeds the power (energy) density specification. See the Definitions tab above for a breakdown of the type of specifications that apply to each sensor type.
Linewidth of Light Sources
When using a thermal or pyro-electric sensor, the linewidth of light sources can be neglected. However, for photodiode sensors, the current generated is strongly dependent on the operating wavelength. If the light source has a linewidth greater than 10 nm (e.g., an LED), the power meter may display an incorrect power reading.
To achieve a reasonable power measurement for broadband light sources when using a photodiode sensor, the detector's response curve must be nearly linear over the linewidth of the source. If the operating wavelength is set on the power meter as the center wavelength of the light source, the measured optical output power will be approximately correct if the spectrum of the light source is symetric around the center wavelength.
The surfaces of photodiodes, neutral density (ND) filters, and even the black coatings of thermal sensors all cause some back reflection of incoming light. If this back reflection enters the aperture of devices such as a laser diode or a HeNe laser, it may impact the laser's power stability. We recommend slightly tilting the power meter sensor with respect to the laser beam so that any back reflection will not enter the output aperture of the laser. If back reflections must be completely avoided, we recommend using one of our integrating sphere sensors, since the incoming light is nearly completely absorbed inside the sensor.
Ambient and Stray Light
Ambient or stray light can strongly affect measurement accuracy in free-space applications. A constant background light level can be subtracted by resetting the zero level of the detector. The power meter cannot compensate for varying ambient light, such as daylight or turning the room light on or off. In these cases, the sensor should be properly shielded from ambient and stray light. Thorlabs offers a large selection of blackout materials that can be used for this application.
Thorlabs' power sensors are all calibrated at room temperature (23 ± 5 °C).
In most cases, the responsivity of a photodiode sensor has a negligible temperature dependency when compared to the overall measurement accuracy of ±5%. However, the dark current is temperature sensitive, which can interfere with measurements of low optical powers. Proper zeroing is recommended to compensate in this case (see your sensor's manual for details).
In constrast, thermal sensors will respond to any temperature differences that occur between the thermal disk and the heat sink. Disturbances in airflow around the sensor or an increase in the temperature of the heat sink, as could occur during a long-term exposure of the thermal disk to a laser beam, can all interfere with obtaining an accurate power measurement. To minimize these effects, the sensor should be shielded from air flow as much as possible. The sensor should also be zeroed properly for the user's operating conditions. For instance, a cold sensor can be zeroed for short measurements. However, for long-duration measurements, the sensor should be allowed to reach a state of thermal equilibrium prior to zeroing (e.g., after ~10 minutes of light exposure).
Reducing Noise for High Accuracy Measurements
Noise from grounding, cable capacitance, temperature effects, stray and ambient light, and the detector can interfere with the accuracy of measurements. These noise sources will have a higher impact when attempting to measure low optical powers. Below are some hints and recommendations to minimize the effects of noise:
Laser light is emitted from an optical fiber tip in a conical shape with an opening angle of twice the acceptance angle of the fiber. The fiber acceptance angle can be calculated by:
where NA is the numerical aperture of the fiber and n is the index of refraction (for air, n=1).
For typical fibers, the total angle (2Θ) of the emitted light is between 15° and 25°. If an angled connector (APC) is used, the cone will be tilted by approximately 4° from the fiber axis.
To obtain an accurate measurement, the expansion of the beam that occurs from the fiber tip to the detector location must be considered to avoid overfilling the detector. Conversely, maintaining a certain size gap between the fiber tip and detector surface is desirable for high-power fiber lasers to decrease the power density.
Thorlabs offers fiber adapters in most common connector styles that are compatible with our standard photodiode power sensors, all-in-one wireless power meter, and most of our thermal sensors. For measurements of the output from fibers with large divergence angles or where avoiding back reflections is critical, we recommend using one of our integrating sphere sensors.
We also offer sensor heads specifically designed for fiber-based measurements. These sensor heads plug directly into the power meter and have no cable between the sensor and the console, minimizing measurement interferences.
Thorlabs' energy sensors are based on the pyroelectric effect and have a thermal time constant of 20 ms. As a result, the energy measurements are limited to a maximum repetition rate of 30 to 40 Hz when the sensor is connected to a 1 MΩ load. Important considerations for obtaining an accurate energy measurement are outlined below.
A pyroelectric sensor delivers a voltage pulse with a peak amplitude proportional to the pulse energy. When the power meter console is operating in peak detector mode, it indicates the energy of each individual pulse. The trigger level can be set between 0 and 99% of the selected energy range and defines the lower threshold at which the peak detector recognizes the pulse, begins recording data, and waits until the peak is reached. Consequently, the trigger level must be set higher than the noise level and lower than the expected energy. A useful method is to connect the sensor, select the desired range, and increase the trigger level until the console ceases to measure the power continuously. Please note that the last measurement value remains displayed (peak-hold in energy measurement mode).
The pulse repetition rate must not exceed the maximum value specified for the actual sensor type. Otherwise, the measured value will be affected by the subsequent pulse.
Increasing the Maximum Repetition Rate
The only way to increase the maximum measurable repetition rate is to use the energy sensor with an oscilloscope (high-Z input) and a 100 kΩ termination resistor (Note: We typically provide a calibration value for our energy sensors in this configuration. Lower voltages may be used to further lower the repetition rate, but this will weaken the signal to the oscilloscope. Data for sensor performance at lower resistances is not provided in the individual sensor calibration data sheets.)
To accomplish this setup with our C-series pyroelectric sensors, first disconnect the BNC connector from the red D-sub connector. Connect the BNC connector via the 100 kΩ terminator to the high impediance Y input of the oscilloscope. The responsivity in J/V of the energy sensor with 100 kΩ termination can be found in the specification sheet for each individual sensor and can be used to calculate the power from the measured peak voltage.
Energy sensors can detect and measure pulses with a duration from the sub-nanosecond range to approximately 2 ms. The maximum pulse duration depends on two factors:
Usually, the thermal time constant has a more significant effect.
Simple Function Test
An energy sensor not only responds to optical pulses incident on the absorber but also to mechanical pulses as well. This feature can be used to quickly check the operability of a pyroelectric sensor.
A power meter can read the average power of a pulsed signal under the following conditions:
Using a Thermal Sensor
The peak power is lower than the damage threshold of the sensor. This is true regardless of pulse length and repetition rate. As thermal sensors react very slowly, they will integrate the power incident on the active area.
Using a Photodiode Sensor
A photodiode sensor can differentiate short pulses in the nanosecond range. Therefore, the pulse peak power should not be larger than the maximum power range of the sensor. If the pulse peak power exceeds the set power range, the reading will clip and produce an incorrect average value. The Min-Max display function can be helpful for finding the appropriate power range.
The bandwidth setting will also influence the power reading for certain pulse lengths and repetition rates. Use the 'LOW' bandwidth setting for a stable display and the 'HIGH' bandwidth setting to monitor the pulse via the analog output.
The repetition rate of femtosecond lasers is up to several hundred MHz, while the the pulse duration is on the order of tens of femtoseconds. Thermal sensors usually provide the best results for measuring the average power, since the energy of the pulses is usually moderate. However, it is still important to verify that the source won't exceed the maximum ratings for the selected sensor. Definitions of the sensor specifications and common formulas for calculating pulse characteristics can be found on the Definitions tab.
Is the Wavelength Setting Correct?
Make sure that the wavelength entered in the console matches the wavelength of the incident light source. An incorrect wavelength setting can cause large inaccuracies in the power reading. Also check the accuracy and precision to which the wavelength of your incident light source is known.
Example: An S120C photodiode power sensor is used to measure a light source with a stated wavelength of 405 nm. However, the light source actually has an emitted wavelength of 401 nm, resulting in a a 15% error in the measured power.
When was the sensor last calibrated?
The responsivity of a sensor will change over time. Photodiode sensors or sensors used in the UV range are particularly susceptible to this effect. We recommend recalibrating our sensors annually. More information about how to order recalibration for your sensor can be found on the pages with the full presentation for each sensor (see the Sensor Selection tab for links).
Is the power of the incident light within the specified operating range?
The specifcations for your sensor are included in the Specifications Sheet. Be careful not to confuse the "Resolution" specification with the minimum measurable power (the resolution is the minimum detectable change in power and is equal to the least significant digit on the console's display when the console is set to the lowest measurment range).
Example: The S120C is specified as having an optical power range between 50 nW and 50 mW and a resolution of 1 nW. This means that the minimum measurable power is 50 nW and that the least significant digit on the display for this measurement range will have a value of 1 nW.
How well was the sensor zeroed?
Photodiode sensors will emit a low level current even if no photons are incident on the active area, referred to as the sensor's dark current. The dark current, which is temperature dependent, will generally be on the order of a few nA for Si and InGaAs sensors and up to a few µA for Ge sensors.
Thermopile sensors need to be zeroed if thermal differences appear between the active area (thermal disk) and the sensor heat sink in the dark state (no incident light upon the active area) or in the case that the heat sink becomes hot during light exposure. The voltage generated by a thermopile with no light incident on the active area will be negative if the heat sink is hotter than the active area and positive if the heat sink is cooler than the active area. When both the heat sink and detector are at room temperature, a voltage of a few µV is typical.
The effects of dark current can be avoided by correctly zeroing the sensor. Detailed instruction can be found in the manual for you sensor head.
Can the maximum repetition rate of an energy sensor be increased?
Yes. The maximum measurable repetition rate can be increased by using the energy sensor with an oscilloscope (High Z input) and a 100 kΩ termination resistor. Disconnect the BNC connector of the sensor from the red D-sub connector; then, connect it via the 100 kΩ terminator to the high impedance Y input of the scope. The responsivity [J/V] of the energy sensor for 100 kΩ termination can be found in the product-specific data sheet. The energy can then be calculated from the measured peak voltage.
Is the specified maximum optical power of the photodiode and integrating sphere sensors related to CW or pulsed power?
The maximum power range is specified for use with CW sources. For pulsed light sources, the pulse peak power must not exceed this value in order to avoid saturation of the photodiode.
Can I use the photodiode-based sensors for pulsed measurements?
Under certain conditions, the average power of pulsed light sources can be measured. The pulse peak power must not exceed the specified maximum optical power to avoid saturating the sensor. Please refer to the Measurement Considerations/Power Measurement of Pulsed Signals section of the power meter manual or the appropriate trouble shooting topic for further information.
How does an integrating sphere work? Can I use it for measurements of pulsed lasers?
See the Overview tab to learn how an integrating sphere functions. Integrating spheres can be used for pulsed signal measurements under the same conditions as photodiode sensors.
My ES1xxC or ES2xxC sensor does not show any energy. Which settings do I need to change? What is the definition of the trigger level?
Check if the measurement range and trigger level are set correctly. The trigger level defines the minimum measureable pulse energy and is required to suppress sensor noise. If you are measuring a pulsed signal, check if the sensor is capable of detecting signals with your source's repetition rate and pulse length.
|Sensor Type||Parameters Indicating Damage Thresholds|
|Photodiode (Except Integrating Sphere)||Maximum Average Power Density
Maximum Pulse Energy
|Integrating Sphere (Photodiode)||Maximum Average Power Density
Maximum Pulse Energy
Maximum Intermittent Power (2 min)
|Pyroelectric Sensor||Maximum Power Density (Peak Power)
Maximum Pulse Energy Density
Maximum Average Power
How is the damage threshold defined? Do I need to use the peak power value or the average power value?
When measuring pulsed signals, the damage threshold depends on a variety of parameters such as pulse duration, beam diameter, and power (pulse energy). The maximum ratings given for our power and energy sensors should not be exceeded, particularly those listed for the maximum power or energy densities. Exceeding these values might damage the sensor.
The "Maximum Intermittent Power (2 min)" specification is given for sensors capable of measuring high powers. This value is specified above the upper limit of the measurement range; consequently, the sensor will not give correct power readings if used with these high intermittent powers. However, the sensor will not be damaged if it is exposed to these power levels for less than 2 minutes. In the case of pulsed signals, the average power of the pulse must not exceed this specification. Please refer to the Definitions tab to see how these parameters are calculated.
Which sensors can I use for measuring femtosecond pulses?
A thermal sensor will provide the best results when measuring the average power of a femtosecond laser pulse.
How can I increase the bandwidth of an ES1xxC or ES2xxC sensor?
If we assume that the bandwidth is equivalent to the maximum pulse repetition rate, this parameter can be altered by changing the input resistance of the measurement device. In the case of a power meter console, the input resistance is fixed at 1 M? and the maximum repetition rate cannot be changed. In order to increase the repetition rate, see the Sensor Tutorial tab for how to accomplish this using an oscilloscope and a 100 k? termination resistor.
What is the difference between resolution and minimum optical power?
The resolution is the minimum detectable change of power. The minimum optical power is given by the optical power range of the sensor. Please see the Tutorial tab for an example.
|Wavelength Range||The range of wavelengths over which the sensor is calibrated.
The specified measurement uncertainty only applies for this wavelength range.
|Resolution||This is the minimum detectable change of the measured parameter. Resolution is always specified for a certain console type and bandwith setting, which is typically given in the footnotes of the specifications table on the sensor's spec sheet.||All Sensors|
|Measurement Uncertainty||This parameter indicates the measurement accuracy and is generally specified for the entire wavelength range of the sensor. For certain sensor types, several values may be specified (on the specification sheet), with different values corresponding to different regions of the specified wavelength range.||All Sensors|
|Optical Power Range||The optical power range lists the minimum and maximum measurable power that the sensor can detect. Exceeding the upper limit leads to sensor saturation and incorrect measurement results. When measuring pulsed signals, the peak power of the pulse must not exceed the maximum measurable power to avoid saturation. Attempting to use a sensor to acquire power measurements below the lower limit will increase the measurement uncertainty due to the effects of noise.||Photodiode, Integrating Spheres, Thermal|
|Optical Energy Range||Specifies the minimum and maximum measurable energy. Care should be taken when setting the trigger level to measure pulses with energy levels close to the lower limit of the optical energy range. Sensor noise may interfere with the pulse edge triggering the sensor correctly, causing incorrecte measurement results.||Pyroelectric|
|Max Average Power Density||The average power per unit area that can be incident on the sensor without causing damage. This value must not be exceeded to avoid damaging the sensor. Equations for calculating the average power density of a light source with a square pulse train and circular beam shape are given below in the Calculations section.||Photodiode, Integrating Spheres, Thermal|
|Max Pulse Energy||An alternative specification to the max average power density. The total energy in a pulse must not exceed this value to avoid damaging the sensor.||Photodiode|
|Max Pulse Energy Density||Another alternative specification to the max average power density. The maximum energy density a pulse can have before causing damage to the sensor. Besides thermal sensors and integrating spheres, we provide this specification for our fiber power sensors. Fibers may have a very small beam diameter at the tip, leading to high energy densities that could damage the sensor if they are above this specification.||All Sensors|
|Max Intermittent Power
(2 minutes Max)
|The maximum power that can be applied to the sensor for less than 2 minutes without causing damage. For pulsed signals, this value should be compared with the average power of the pulse train. This specification is higher than the maximum measurable power, so the sensor will be saturated at this value and provide an incorrect measurement.||Integrating Spheres, Thermal|
|Max Power Density||This value is related to the peak power of the incident pulse and should not be exceeded to avoid damaging the sensor.||Pyroelectric|
|Max Average Power||The maximum average pulse power that can be incident on the sensor without overheating it. Equations for calculating the average power of a light source with a square pulse train and circular beam shape are given below in the Calculations section.||Pyroelectric|
Measurement Range - The ranges that the power meter console can display power or energy readings. The optical power or energy measurement ranges for Consoles are stated in terms of current (photodiode sensors) or voltage (thermal and pyroelectric sensors). How the current or voltage is converted to a power or energy measurement is sensor dependent.
Display Resolution - The minimum detectable change in power or energy that can be shown on the power meter console's display screen. These values are also given in terms of current or voltage, as this is the input that the console measures. The optical power resolution will depend on the responsivity of the detector connected to the console.
Current Input Resolution - This specification applies to photodiode sensors and is generally specified in terms of current per responsivity value, e.g. "1 pA / Responsivity Value (A/W). This means that the console has a resolution of 1 pA in its lowest measurment range. Note at this is not the minimum measurable power. As the responsivity of photodiode sensors is wavelength dependent, the power resolution of the console will be wavelength dependent as well.
Example: An S120C sensor has a responsivity of 7.35 × 10-2 A/W at 930 nm, and 5.05 × 10-3 A/W at 455 nm. In the lowest measurement range the displayed power resolution is at 930 nm given by
1.1 x 10-12 A/(1.36 x 10-11 A/W) = 13.6 pW and at 455 nm by 1.1 x 10-12 A/(5.05 x 10-3 A/W) = 198 pW. Note that this is not the same as the minimum measurable optical power for the S120C of 50 nW.
Voltage Input Resolution - Typically, power and energy meters will have the voltage resolution specified separately for thermopile and pyroelectric sensors. For thermopile sensors, this value is specified as volts per responsivity value in V/W, e.g. 1 µV/Responsivity Value (V/W). As with photodiode sensors, the power resolution will depend on the responsivity of the sensor. For pyroelectric sensors, the resolution will be specified as volts per responsivity value in V/J, e.g. 100 µJ/ Responsivity Value (V/J). Again, the energy resolution is sensor dependent.
Accuracy - The power meter accuracy is determined by the voltage or current measurement accuracy. This value is typically given as a percentage of the full scale value (% f.s.). The console's accuracy is a different specification than the sensor's measurement uncertainty.
Wavelength Correction - A value that can be entered into the console in order to apply the correct responsivity so that the correct power or energy reading calculated and displayed. Thorlabs calibrated sensors have a responsivity calibration tables stored in their connectors. Our consoles can access this information to perform the power or energy calculation. For custom sensors, the console needs to be set to the correct sensor type (photodiode, thermal, or pyroelectric) and the responsivity will need to be entered as a numerical value.
The following equations can be used to quickly convert the parameters defining a square pulse train into values that can be compared to the specifications of our sensors. The first table outlines how use the pulse duration and pulse period to calculate the repetition rate and duty cycle, how to convert between average and peak power, and how to use the power to determine the total energy in your pulse. The second table describes how calculate the power and energy densities for a circular beam with a square pulse train using these parameters. Making sure that the power density and/or energy density of your source are within the specifications of the sensor is important in order avoid damaging the sensor head.
|Repetition Rate: fP|
|Duty Cycle: D|
|Pulse Average Power: Pav|
|Pulse Peak Power: Ppeak|
|tP: Pulse Duration||TP: Pulse Period|
|Ppeak: Pulse Peak Power||Pav: Pulse Average Power|
The diagram above outlines the defining properties of a square pulse train.
Power and Energy Density
Power density and energy density are defined as the power or energy per unit area of a beam, respectively. The following formulas are based on a circular beam shapre with a beam diameter dB and an area of
For non-circular beam shapes, other forumlas must be used to calculate the beam area.
The average power density is the ratio of the average power of the light beam to the area illuminated by the beam.
|Pulse Characteristic||Known Pulse Parameters||Formula|
|Average Power Density: Ψav||Average Power: Pav
Beam Diameter: dB
|Pulse Peak Power: Ppeak
Repetition Rate: fP
Pulse Duration: tP
Beam Diameter: dB
|Pulse Energy: W
Repetition Rate: fP
Beam Diameter: dB
|Peak Power Density: Ψpeak||Pulse Peak Power: Ppeak
Beam Diameter: dB
|Pulse Energy: W
Pulse Duration: tP
Beam Diameter: dB
|Average Power: Pav
Repetition Rate: fP
Pulse Duration: tP
Beam Diameter: dB
|Pulse Energy Density: ξ||Pulse Energy: W
Beam Diameter: dB
|Pulse Peak Power: Ppeak
Pulse Duration: tP
Beam Diameter: dB
|Average Power: Pav
Repetition Rate: fP
Beam Diameter: dB